1. Field of the Invention
This invention relates to an anti-skid control apparatus for a vehicle braking system which can prevent locking of the wheels.
2. Description of the Prior Art
For example, the anti-skid control apparatus shown in FIG. 1 is known.
In FIG. 1, a brake pedal 2 is connected to a tandem master cylinder 1. One fluid pressure chamber of the tandem master cylinder 1 is connected to a wheel cylinder 7a of a right front wheel 6a through a conduit 3, an electro-magnetic three position valve device 4a and a conduit 5. The conduit 5 is further connected to a wheel cylinder 12b of a left rear wheel 11b through a conduit 13 and a proportioning valve 32b.
Another fluid pressure chamber of the tandem master cylinder 1 is connected to a wheel cylinder 7b of a left front wheel 6b through a conduit 16, an electro-magnetic three position valve device 4b and a conduit 17. The conduit 17 is further connected to a wheel cylinder 12a of a right rear wheel 11a through a conduit 15 an a proportioning valve 32a.
Discharge openings of the valves 4a and 4b are connected through conduits 60a and 60b to hydraulic reservoirs 25a and 25b, respectively. The hydraulic reservoirs 25a and 25b include pistons 27a and 27b slidably fitted to a casing and relatively weak springs 26a and 26b. Reserve chambers of the reservoirs 25a and 25b are connected to suction openings of a fluid pressure pump 20.
Although the fluid pressure pump 20 is schematically shown, it consists of a pair of casings 21, pistons slidably fitted to the casings 21, an electro-motor 22 reciprocating the pistons, and check valves 23a, 23b, 24a, 24b. Supply openings of the fluid pressure pump 20, or the sides of the check valves 23a, 23b are connected to the conduits 3 and 16.
Wheel speed sensors 28a, 28b, 29a and 29b are associated with the wheels 6a, 6b, 11a and 11b respectively, and they generate pulse signals having frequencies proportional to the rotational speeds of the wheels 6a, 6b, 11a and 11b. The pulse signals of the wheel speed sensors are supplied to a control unit 31.
The control unit 31 has a well-known circuit construction. It generates control signals Sa, Sb and a motor drive signal Qo. The control signals Sa, Sb are supplied to solenoid portions 30a and 30b of the valves 4a and 4b respectively. Dashed lines represent electric lead wires.
Although the electromagnetic valves 4a and 4b are shown schematically, their construction is well known. When the level of the control signal is "0", the valves take first positions A for increasing the brake pressures at each wheel. In the first positions A, the master cylinder side and the wheel cylinder side are connected. When the level of the control signals is "1", the valves take third positions C for decreasing the brake pressure to the brakes. In the third positions C, the connection between the master cylinder side and the wheel cylinder side is interrupted, while a connection is made between the wheel cylinder side and the reservoir side. The brake fluid is discharged through the conduit 60a and 60b into the reservoir 25a and 25b from the wheel cylinders 7a, 7b and 12a and 12b. When the level of the control signal is "1/2", the valves 4a and 4b take second positions B. The communications between the master cylinder side and the wheel cylinder side and between the wheel cylinder side and the reservoir side are interrupted. Thus, the fluid pressure to the brakes is kept constant.
The control unit 31 further generates the drive signal Qo for the motor 22, which signal continues during the skid control operation.
Generally in the arrangement described above arrangement, the braking forces are so distributed that the front wheels 6a and 6b tend to lock sooner than the rear wheels 11a and 11b. Accordingly, the control signals Sa and Sb become "1" for relieving the brakes of the front wheels 6a and 6b. The change-over valves 4a and 4b are changed over into the position C. The pressurized fluid is discharged into the reservoirs 25a 25b from the wheel cylinders 7a and 7b. Accordingly, the brakes of the front wheels 6a and 6b are relieved. However, at the same time, pressurized fluid is discharged into the reservoirs 25a 25b also from the wheel cylinders 12a and 12b of the rear wheels 11a 11b. Thus, also the brakes of the rear wheels 11a and 11b are relieved, although the rear wheels 11a 11b have not yet tended to lock. Thus, the rear wheels 11a 11b are subject to under-brake. The word "under-brake" means that the braking force is insufficient.
However, when the above assumption is not fulfilled, for example, when only the front wheels are provided with spiked tires or chains for running on snow or ice, and the rear wheels are provided with the normal tires, the rear wheels tend to lock sooner than the front wheels.
Even when the front and wheels are provided with tires of the same kind, the rear wheels may tend to lock sooner than the front wheels, when the frictional coefficient of the brake lining becomes excessively low due to thermal fade phenomenon in a front wheel brake apparatus and the limit lock pressure of the front wheel becomes excessively high.
In the above cases, the front wheels are subject to under-braking.
Even when the braking forces are equally distributed for the front wheels 6a 6b and the rear wheels 11a 11b, there is the following problems: When the so called "select low control" is effected between the front and rear wheels of the same conduit system on the road which is frictionally different at both sides, defined as a split road, the front or rear wheel on the high-.mu. (frictional coefficient) side of the road is subject to a remarkable under-brake.
In the "select low control", the change-over valves are controlled with the skid condition of the front or rear wheel on the one side of the road which is frictionally lower than the other side of the road. Or when the "select high control" is effected between the front and rear wheels of the same conduit system on a split road, the front or rear wheel on the low-.mu. side of the road is subject to locking. Thus, the running stability of the vehicle is lost. That is dangerous.
In the above embodiment, there are only two change-over valves 4a and 4b. Accordingly, the cost is relatively low, but in any case, under-braking cannot be avoided.
Of course, when four three-position change-over valves are provided for all of the wheels, respectively for controlling them independently, there is no problem. However, the three position change-over valve is complicated in construction and it is expensive. Accordingly, the cost is significantly higher.